The present invention relates to a method for the preparation of a frame-supported pellicle for photolithography or, more particularly, a frame-supported pellicle used in photolithography for dust-proof protection of a photomask in the manufacture of semiconductor devices such as LSIs, VLSIs and the like and liquid-crystal display units in a procedure of pattern-wise exposure to light having a wavelength of 500 nm or shorter.
As is well known, a process of photolithographic patterning is involved in the manufacture of various kinds of electronic devices including semiconductor devices such as LSIs, VLSIs and the like as well as liquid crystal display units, in which a semiconductor silicon wafer or a base plate for liquid crystal display units is exposed pattern-wise to light through a transparency of a photomask bearing the desired fine pattern. One of the serious problems in this photolithographic process is that sometimes dust particles are deposited on the photomask to greatly affect the quality of the pattern which should be a high-fidelity reproduction of the pattern on the photomask.
Needless to say, the photolithographic patterning works are usually conducted in a clean room but the above mentioned problem due to deposition of dust particles on the photomask cannot be completely solved even by working in a clean room of highest cleanness. Accordingly, a method proposed in this regard is that a frame-supported pellicle, which is a highly transparent membrane of a plastic resin, is mounted on the photomask and the photolithographic pattern-wise light-exposure is conducted through the pellicle membrane which serves as a dust-proof protector for the photomask.
Since the frame-supported pellicle is used for the dust-proof protection of the photomask, it is a matter of course that the pellicle is absolutely free from dust particles or foreign materials deposited thereon although it is an extremely difficult matter to ensure perfect cleanness of a pellicle without deposition of foreign materials by a variety of reasons leading to the occurrence of particles including the particles floating in the atmosphere of the manufacturing line and clean room, particles contained in the pure water used for rinse and the like and particles contained in the starting materials of the pellicle membrane per se and in the adhesive used for adhesive bonding of the pellicle membrane and the frame supporting the membrane.
It is understood that the process of mechanical working for the preparation of the pellicle membrane is the most responsible for the occurrence of dust particles. For example, a frame-supported membrane is prepared usually by adhesively bonding a separately prepared pellicle membrane to a rigid frame followed by trimming of the membrane to cut off the extraneous portion of the membrane protruded out of the frame. When this trimming work of the membrane is conducted by using a bladed cutter or a punch, tiny fragments of the plastic resin film are unavoidably formed and eventually deposited on the pellicle membrane and pellicle frame. Once such foreign particles are deposited on the membrane, it is almost impossible to subsequently remove them. Another problem in the above mentioned mechanical trimming work is that, since the trimming work is conducted on a pellicle membrane which is formed on a substrate plate by the method of casting of a solution of the plastic resin thereon and the like, the substrate plate is eventually damaged by the trimming punch or other cutting tools for the mechanical trimming work.
In view of the above described disadvantages in the mechanical trimming method of pellicle membranes, a proposal is made for the use of a laser beam which in principle cuts off the pellicle membrane by melting the thermoplastic resin film forming the pellicle membrane so that the problem of occurrence of cutting debris or dust particles can be solved, which may otherwise be unavoidably deposited on the pellicle membrane. The trimming method by using a laser beam, however, is not free from a serious problem. Namely, pellicle membranes are required to be suitable for use in the photolithography in which the light for pattern-wise exposure is the i-line or an excimer laser beam of very short wavelengths. This means that the pellicle membrane is necessarily formed from a thermoplastic resin having high transmissivity to a short wavelength light down to the so-called vacuum ultraviolet such as the KrF and ArF excimer laser beams having a wavelength of 248 nm and 193 nm, respectively. Accordingly, the pellicle membrane formed from such a resin having high transmissivity to the laser beam hardly absorbs the energy of the laser beam used for the trimming work of the pellicle membrane such as the beams from YAG lasers or above mentioned excimer lasers so that laser beams are not suitable for the trimming work. Even if a pellicle membrane could be trimmed by using a laser beam, another problem in the use of a laser beam is that the substrate plate, on which the pellicle membrane is formed and supports the membrane during the trimming work, made from fused quartz glass or silicon would also be damaged by the laser beam. Therefore, the only practical method used for the trimming work of pellicle membrane is the mechanical method despite the problems that the method is accompanied by a decrease in the quality of the pellicle membrane as well as in the yield of acceptable products.